1. Field of the Invention
The present invention relates to the field of radar detection and tracking systems.
2. Prior Art
In conducting a radar search for airborne targets, it is conventional practice to use a narrow antenna beam. For fast-moving targets, especially at short range, the surveillance volume must be rapidly scanned, which leads to a short dwell time on a target. On the other hand, for tracking and discriminating targets, a much longer dwell is needed. These conflicting requirements have traditionally led to an expensive system based on a phased array antenna, or to multiple systems designed specifically for each function. Such expense has been justified in conventional warfare where the general location of the enemy is known, the engagement is fairly static, and the equipment is not in harm's way. However, such systems are no longer appropriate for modern warfare, such as combating insurgents, where an attack can occur at short range, from any direction, and without warning.
The basic requirement for target detection in any radar application is that the return signal must contain sufficient energy relative to the noise power density. This can be accomplished with some combination of high transmit power, an antenna with high gain, and use of a long dwell time on target in the detection process. Range resolution is also an important factor, not necessarily for detection purposes, but for clutter suppression and target analysis (discrimination, classification, or identification). Much of radar system design consists of making trades among these parameters.
There is a practical limit to the transmit power, as increasing power beyond some relatively low level has a disproportionate impact on system cost and reliability. Until now, there has also been a practical limit to the dwell time for detecting targets in clutter, as in the prior art, the target had to be considered essentially stationary within a range gate and Doppler filter during this time. The only item that offered much flexibility in the prior art design is the antenna. However, increasing the antenna gain has two effects: (1) the antenna will be larger, more expensive, and more difficult to transport and setup; and (2) the beam will be narrower, providing less time on target for a given time to scan the surveillance volume.
A slow scan rate might be acceptable for long-range targets, but not for those at short range, and a short dwell time on target combined with low resolution in range might be acceptable for detection purposes, but not for target analysis. Because of mechanical inertia, the fast scan rate and long dwell time are incompatible for a radar that employs a conventional reflector antenna. One solution would be to use separate antennas (or separate radars) for search and track, but mechanical inertia in the antenna again limits the number of widely dispersed targets that can be tracked at any one time. Under stressing conditions one could theoretically devote additional radars to this task, but a more practical approach is to use a phased array antenna, which can interleave tasks without having to be physically rotated.
Phased arrays, however, are typically large, immobile, and very expensive. Such expense has been justified in conventional warfare, where the action takes place at long range and in a sector no wider than about 120°. Three such arrays would be required to cover all directions, which is a tripling of the size and cost of an already large and expensive system. Such systems are totally unsuitable for modern warfare, where targets can appear suddenly at short range, and from any direction.